Have you tried sitting quietly in the car and not going anywhere? This method takes about a week, I have done this with a dog that hated getting in the car, after trying this one out we couldn't keep her out of the car. LOL Here is a link that explains it well. I would do this gradually, and don't rush it.
Or I would ask your vet for a mild tranquilizer. I have used Acepromazine in the past and it worked out fine. If you wish to try the holistic way, try "Rescue Remedy". I have never used RR but some think it is very good.
For car-sickness, we give our 15 pound dogs:
1/4 tablet Dramamine or generic motion sickness tablets 50 mg tablet(dimenhydrinate/antiemetic)
12.5 mg of medication.
Cream cheese is a great way to encourage the dog to ingest the imbedded medication.
For 30-40 pound dogs: 1/2 tablet
An easy way to cut the tablets:
Sharp paring knife or single edged razor blade